Like gasoline engines, diesel engines have been widely used for transportation and stationary applications. A combustion exhaust from diesel engines often contains a variety of combustion waste materials including unburned hydrocarbon (HC), carbon monoxide (CO), particulate matter (PM), nitric oxide (NO), and nitrogen dioxide (NO2), with NO and NO2 collectively referred to as nitrogen oxide or NOx. Removal of CO, HC, PM, and NOx from the combustion exhaust is needed for cleaner emissions. The combustion exhaust treatment becomes increasingly important in meeting certain emission requirements.
Conventional emission control systems often use selective catalytic reduction (SCR) catalyst for the reduction of NOx. Urea SCR technology using base metal containing catalysts, in particular, is under development to reduce NOx emission from lean gasoline and diesel engine exhaust gas. Efficient NOx reduction is needed in a relatively broad temperature range, for instance at low temperatures during cold start events and at high temperatures during diesel particulate filter (DPF) events and lean gasoline highway conditions.